1. Field of the Invention
This invention is related to insulated glass design for use in building exterior wall systems such as curtain wall, window wall, and punch-out window.
2. Description of the Background
An insulated glass unit, commonly known in the industry as an IG unit, comprises a perimeter spacer bar sandwiched between an exterior single glass pane and an interior single glass pane. The two glass panes are bonded and air sealed to the spacer bar normally with shop-applied silicone caulking. The spacer bar is located inwardly away from the edges of the glass at a small distance, normally about 2 to 4 mm, to create a complete loop-around perimeter channel for additional shop-applied air seal caulking around the entire perimeter of the IG unit. The above shop sealing method is intended to form a long-lasting perfect seal around the IG unit.
The air trapped inside the IG unit provides two important performance functions, namely, thermal insulation and wind load transfer between the exterior glass pane and the interior glass pane. For a positive wind load acting on the exterior glass pane of the IG unit, the dead air space within the IG unit will be in compression, creating a positive differential air pressure relative to the interior air. The differential air pressure will push the interior glass pane inwardly, allowing the sharing of the positive wind load. For a negative wind load acting on the exterior glass pane of the IG unit, the dead air space within the IG unit will be in expansion, creating a negative differential air pressure relative to the interior air. This differential air pressure will push the interior glass pane outwardly, allowing the sharing of the negative wind load.
It is well known in the industry that the shop-applied perfect perimeter seal around the IG unit will inevitably fail in the form of hair line cracks due to stresses caused by relative structural movements and/or aging of the sealant material. For example, perimeter seal failures are often experienced when IG units are shipped over mountainous areas due to air pressure changes in combination with the effect of structurally unrestrained perimeter edges of the IG units. For the in-service condition of the IG unit (i.e., being glazed in a wall system), perimeter seal failure is normally identified from observation of a change in certain performance functions. The noticed change in the performance functions is unacceptable to building owners and the only repair is to replace the failed IG unit.
The following terminology regarding an IG unit will be used: (1) Face 1: the outside-facing surface of the exterior glass pane; (2) Face 2: the inside-facing surface of the exterior glass pane; (3) Face 3: the outside-facing surface of the interior glass pane; (4) Face 4: the inside-facing surface of the interior glass pane. The terms “outside-facing” and “inside-facing” refer to the direction the glass pane faces, when the IG unit is installed, relative to the building exterior and interior. Thus, “outside-facing” refers to the surface facing the building exterior and “inside-facing” refers to the surface facing the building interior.
It can be readily understood that similar to a flat tire condition, if the perfect seal around the IG unit becomes imperfect allowing air leakage, the structural function of the IG unit will be impaired. This change of performance function is not noticeable in the early stages of seal failure.
The effect of a hair line crack in the perimeter seal of an IG unit in a conventional exterior wall system is explained as follows. The IG unit is normally glazed into the wall system frame by using a double air-seal system (i.e., air seal around both the exterior and the interior perimeters of the IG unit). The glazing air seal can never be considered a perfect seal, and moisture migration through the glazing air seal is inevitable. Due to vapor pressure, the moisture migration will be in the direction from a high absolute humidity zone to a low absolute humidity zone. For example, in a heated and humidified building during the winter time, interior moisture will migrate through the interior glazed seal line into the joint cavities around the perimeter edges of the IG unit, then through hair line cracks in the perimeter seal into the dead air space of the IG unit, causing water condensation on Face 2 of the IG unit. Similarly, in an air-conditioned building during a humid summer day, exterior moisture will migrate through the exterior glazed seal line into the joint cavities around the perimeter edges of the IG unit, then through hair line cracks in the perimeter seal into the dead air space of the IG unit, causing water condensation on Face 3 of the IG unit. This kind of change in performance function is immediately noticeable (commonly known as a fogged IG unit) and unacceptable to the building occupant. The only solution is to replace the impaired IG unit.
Another example of change in performance function caused by water infiltration is explained as follows. When exterior rain water infiltrates through the exterior glazed seal line, due to the surface tension of a water drop, the infiltrated water can run slowly along the perimeter seal surface and in contact with hair line cracks in the perimeter seal. During positive dynamic wind load cycles, water may infiltrate through the hair line cracks into the dead air space of the IG unit as explained below. The deformation of the exterior glass pane under a positive wind load will cause an air exhalation from the dead air space of the IG unit through hair line cracks in the perimeter seal. Once the positive wind load starts to recede, a negative pressure is created inside the dead air space causing the water running over a portion of a hair line crack to be sucked into the dead air space of the IG unit. Once the water has infiltrated into the dead air space, it will flow downwardly and accumulate on top of the bottom segment of the spacer bar. Even if some hair line cracks exist in the bottom perimeter seal, the surface tension of a water drop prevents water drainage through the hair line cracks. Therefore, dynamic positive wind load cycles create a one-way pumping action that sucks water into the dead air space of the IG unit. This problem has been observed in curtain wall projects with large IG units due to the fact that the larger the size of the IG unit, the bigger the pumping force. This kind of change in performance function is immediately noticeable and unacceptable to the building occupant, and the only solution is to replace the impaired IG unit.
A major cause of the above-discussed problems is the requirement of a perfect seal design using caulking along the perimeter of the IG unit. It is desirable to eliminate the discussed functional performance problems by having an IG unit design that does not require a perimeter caulking seal.